The invention concerns the technical sector of orthopaedic implants, in particular implants for the cervical spine.
It is known that degeneration of a cervical disc between two vertebrae causes the disc to slip, which generally results in neuromuscular problems after a nerve root of the spinal column becomes trapped. In attempting to overcome these problems, different types of implants have been proposed which may or may not allow the mobility of the vertebrae in question to be preserved. More particularly, the invention relates to implants effecting arthrodesis of the vertebrae.
Among these known solutions, mention may be made of intersomatic cages, intercervical prostheses and rigid mechanical devices which essentially employ a plate and screws.
In the latter case, the plate is positioned on the anterior face of the vertebral bodies to be joined, the screws being engaged through the plate so as to be screwed into a part of the vertebral bodies with a view ensuring that the fixture is held securely in place. It should be noted that a graft can be arranged between the two vertebral bodies, the function of which graft is to ensure mechanical support until bone fusion between the two vertebral bodies takes place. The plate therefore ensures that the graft is held in place and serves as a prop or stiffening element between the two vertebral bodies.
It should be noted, however, that this fixture is relatively complex and its use requires a certain degree of dexterity. Furthermore, it does not allow a constant compression force to be exerted throughout the phase of fusion in order to reduce the healing time.
It has also already been proposed, for example in U.S. Pat. No. 5,395,372, to use arthrodesis plates comprising pointed branches which are intended to be impacted in the vertebral bodies and whose geometry, appropriately inclined, makes it possible to ensure a certain compression force caused by the impaction. These plates are held in place by means of screws. These solutions are unsatisfactory, however.
A retaining clamp for osteosynthesis has also been described in FR-A-2 694 696, made of a shape-memory material and comprising two branches and a crosspiece, it being possible for the branches to be introduced in a rectilinear manner into drill holes before they deform in order to apply compression when the temperature of the clamp recovers the body temperature. Likewise, in application EP-A-0 646 353, it has already been proposed to use clamps made of wire folded back in such a way as to form a plurality of branches which are able to close together by means of a deformation of a shape-memory alloy.
Since the clamp is made of wire, the compression forces or clamping forces possible are dependent on complex processes for obtaining the shape memory, and the restoring forces of the different branches are difficult to manage.
The object of the invention is to overcome these disadvantages in a simple, reliable, effective and economical manner.
The problem which the invention proposes to solve is that of providing arthrodesis of the vertebrae by means of a single compact element which is able to afford constant compression between the vertebral bodies in question and to keep such pressure constant, particularly in the area of the spongy bone.
Furthermore, the compact element, which is intended to be put in place via an anterior approach to the cervical spine, in order generally to maintain a position of lordosis, easily risks being extracted unless, as in the prior art, it is fixed by means of screws. The object of the invention is to ensure, without using complementary securing means such as screws, a perfect holding in place of the clamp, even in the case of physiological movement reducing the lordosis effect.
To solve such a problem, a device has been designed and developed for joining at least two vertebral bodies, characterized in that it comprises at least one plate equipped at each end with a pair of anchoring parts which can be introduced substantially vertically, that is to say axially, into parallel seats previously established in the vertebral bodies to be joined, and then, after introduction, can be folded back at an angle towards one another in order to exert a constant compression of the vertebral bodies and to ensure perfect anchoring.
To solve the problem of facilitating the positioning of the plate while at the same time permitting an automatic compression after impaction, each plate and its anchoring parts are made from a shape-memory material, which shape corresponds to a defined and stable geometric shape at a positive temperature and to another geometric shape at a negative temperature.
At very substantially 0xc2x0 C. the anchoring parts are arranged in a substantially vertical plane, whereas at a positive temperature of very substantially 37xc2x0 C. the said parts are automatically folded back and in. It should be noted that the plates are stored at 0xc2x0 C. in a form allowing the anchoring parts to maintain the vertical position.
To solve the problem of permitting deformation of the anchoring parts after impaction, each anchoring part is connected to the ends of the corresponding plate via a central connection zone delimiting two profiled notches in order to permit the deformation of the said parts.
Advantageously, each anchoring part comprises two tabs which are connected at one of their ends via a common branch which comprises the central connection zone.
It will thus be understood that when the plate recovers body temperature, the anchoring parts in the form of tabs can thus tilt at an angle in a sagittal plane in order to exert a permanent compression on the spongy bone of the vertebral bodies of the vertebrae in question, each time around the central connection zone, whilst the two tabs situated on one and the same central connection zone can tilt towards one another by virtue of the presence of the notches, so as to deform in a frontal plane and more precisely close to the horizontal in such a way as to converge, which prevents the extraction of the plate.
The compression force exerted by the two pairs of anchoring tabs and tending to bring two vertebrae closer to one another can be maintained at a value significantly greater than the force tending to bring two tabs of one and the same pair closer to one another at an angle. This can be easily achieved on account of the fact that the central connection zone can have a considerable amount of material subjected to the memory effect whilst the closing together of the two tabs of a single pair can be determined by the smaller amount of material in the area of the notch at the location where the tab joins the central connection zone.
To solve the problem of ensuring perfect anchoring, each tab has on the inside a series of toothed spikes.
The invention is also distinguished by the following characteristics:
the plate has a transverse curvature corresponding very substantially to the anterior face of the vertebral bodies;
the plate has at least one slot for the engagement of one or more securing screws. This slot is preferably a central slot, which can advantageously be circular. It can be internally threaded or otherwise configured so as to receive an element of an ancillary intended to support the plate when it is at its lowered temperature in order to keep it and present it in a well defined position. This passage can additionally be used for the insertion of a spacing element, such as an intersomatic cage or a bone graft or another material intended to facilitate arthrodesis. Finally, particularly in the case of a plate intended to join two vertebrae on either side of a central vertebra, this passage can be used to receive a screw which is screwed into the body of the central vertebra and thereby to ensure effective joining of the three vertebrae with compression of the end vertebrae against the intermediate vertebra.
The plate according to the invention can also have, half way between the two pairs of anchoring parts or tabs, intersomatic reliefs which are intended to penetrate into the intervertebral space upon positioning of the plate and to maintain the height of the articular midline.